The present invention relates to a process for purifying copper contained in machining scraps and wastes, with those melting processes being eliminated which heretofore were believed to be essential in order to prepare suitable anodes for electrolytic refining from such scrap and waste material.
As known, electrolytic copper refining is carried out in cells into which massive anodes are installed which are manufactured by melting copper scraps, and casting the molten mass into suitable moulds, and into which cathodes are also installed, which are constituted by copper sheets or stainless steel sheets and onto which cathodes onto which refined copper is deposited owing to the effect of the electrical field generated between anode and cathode.
The electrolyte is generally constituted by an aqueous solution of copper sulfate containing free sulfuric acid, with the addition of additives in order to obtain a deposit with good characteristics.
The massive anodes of known type suffer from several drawbacks and limitations of practical character: first of all, the anodes, which get consumed, have to be removed after preestablished time periods, with the manufacturing cycle having thus to be interrupted.
Furthermore, the so-said "anodic residues", which constitute from 15 to 25% of the original metal weight, must be melted again, and this is a further burden.
The anodic sludges get often detached from the anodes and settle on the bottom of the electrolytic cell and must be removed from time to time; furthermore, these sludges may disperse throughout the bath and constitute a polluting agent for deposited copper.
It must be remarked, then, that the anodes to be refined must contain limited levels of certain impurities (Pb, Sn, Fe, O) and normally must undergo a refining process, with the consequent scorification of 7-10 parts of copper per each part of impurities to be eliminated.
The presently used refining system with massive anodes of polluted metal displays the feature that the anodic surface is very close to the cathodic surface, with current density value, expressed as A/m.sup.2, consequently being very similar at both electrodes.
It derives that the cathodic current density cannot be increased and therefore, summing-up, the production capacity of the facility cannot be increased beyond certain limits, in order to prevent anodes from being passivated or cathodic deposits with poor quality being obtained.
The presence of sludges which get detached from the anode prevent the use of techniques which increase the copper diffusion coefficient in the cathodic double layer.
Most electrolytic copper refineries presently existing are operated with a cathodic density of about 200 A/m.sup.2 ; only those which have adopted the so-said "PRC" (periodical reverse current) technique can exceed 300 A/m.sup.2.
Summing-up, the refining system with massive anodes suffers from a large number of electrochemical limits, requires melting and thermal refining furnaces, a complex casting system, a complex handling of new anodes and anodic residues.
This type of facility cannot be operated on a small scale for economic reasons, so copper scrap has to be transported from a large number of different generation sites to a small number of large treatment facilities, with high transport costs.